In recent years, in a field of toners (hereinafter may also be referred to as “the toners” merely) for electrostatic image development, suitable developments of electrophotographic apparatus and toners usable therein have been advanced at a high pitch according to requirements from the market.
For example, as a toner meeting the formation of a high-quality image, a toner having a sharp particle size distribution is required because development behavior of individual toner particles is made even, thereby markedly improving the reproducibility of a minute dot. However, it has been difficult to obtain a toner having a sharp particle size distribution according to a conventional pulverization process. On the other hand, an emulsion polymerization aggregation process has been proposed as a production process capable of optionally controlling the shape and particle size distribution of toner particles. According to this process, a dispersion of fine colorant particles and optionally a dispersion of fine wax particles are mixed with an emulsified dispersion of fine binder resin particles, these fine particles are aggregated with stirring by, for example, adding a flocculant or controlling a pH, and the aggregated particles are further fusion-bonded by heating, thereby obtaining toner particles.
Development of a low-temperature fixing toner capable of fixing with small energy is advanced from the viewpoint of energy saving. In order to lower the fixing temperature of a toner, it is necessary to lower the melting temperature and melt viscosity of a binder resin. When the glass transition point (Tg) and molecular weight of the binder resin are lowered for lowering the melting temperature and melt viscosity of the binder resin, however, such a new problem that the high-temperature storage stability of the resulting toner is lowered is caused.
In order to solve this problem, there has been reported a technique that toner particles are provided as those having a core-shell structure for satisfying both low-temperature fixing ability and high-temperature storage stability (see, for example, Patent Literature 1). In other words, a shell layer is formed with fine particles having a high softening point and excellent heat resistance on the surface of each of core particles having excellent low-temperature fixing ability, whereby a toner satisfying both low-temperature fixing ability and high-temperature storage stability can be prepared. In particular, the production of a toner by the emulsion polymerization aggregation process has the advantage that such shape control can be easily conducted.
In recent years, while the speeding-up of printing in copying machines and printers and the expansion of kinds of paper adaptable thereto have been advanced in a production printing region, however, the fact that both low-temperature fixing ability and high-temperature storage stability are satisfied by such a toner of the core-shell structure as disclosed in Patent Literature 1 has come to be difficult.
In order to solve such a problem, a toner making use of a polyester resin as a material of the shell layer has been developed (see, for example, Patent Literature 2). The polyester resin has the advantage that a design for a low softening point is easily made while retaining a high glass transition point compared with a styrene-acrylic resin, so that the polyester resin is used in the shell layer, whereby a toner good in both low-temperature fixing ability and high-temperature storage stability can be obtained.
However, the polyester resin is poor in affinity for the styrene-acrylic resin, so that there is a problem that when the styrene-acrylic resin is used as a binder resin making up core particles, and the polyester resin is used as a shell resin making up a shell layer, difficulty is encountered on the formation of a thin and even shell layer to fail to achieve sufficient high-temperature storage stability. In addition, fusion bonding between the core particles and fine particles to form the shell layer is hard to occur, so that it is difficult to control the shape of the resulting toner particles. Accordingly, it is difficult to prepare toner particles having a smooth surface. As a result, high charge property cannot be achieved. In addition, the toner is stirred in a developing vessel upon continuous printing, thereby causing peeling of the shell layer. As a result, there is also a problem that image noise occurs on an image obtained upon image formation to fail to ensure good image quality.
In order to solve these problems, there has been proposed a toner of a core-shell structure that a urethane-modified polyester resin or/and an acryl-modified polyester resin is introduced into a shell layer from the viewpoint of improving the affinity of the polyester resin for the styrene-acrylic resin (see, for example, Patent Literature 3).
According to such a toner, a shell layer having a uniform surface to some extent can be formed even when the styrene-acrylic resin is used in the core particles.
However, it may still not be said to be sufficient in that the high-temperature storage stability is not satisfactorily achieved when the low-temperature fixing ability of the binder resin is intended to be further improved.
In addition, there has been proposed a toner of a core-shell structure that a polyester-modified vinyl polymer is introduced into a shell layer from the viewpoint of improving the affinity of the polyester resin for the styrene-acrylic resin (see, for example, Patent Literature 4 and Patent Literature 5).
However, in the toner disclosed in Patent Literature 4, excessive affinity for the styrene-acrylic resin is produced because the content of a polyester segment is low, so that a uniform shell layer cannot be formed. In addition, it is difficult to design a toner that can achieve a low softening point while retaining a high glass transition point, so that there is a problem that both low-temperature fixing ability and high-temperature storage stability cannot be satisfied at the same time.
In addition, the resin of the toner disclosed in Patent Literature 5 has a structure that a styrene-acrylic polymer is graft-polymerized on a main chain by the polyester, so that intramolecular crosslinking occurs in the course of synthesis of said resin, and difficulty is encountered upon controlling a molecular weight of a styrene-acrylic polymer segment. Accordingly, the toner whose shell layer is formed by such a resin can still not satisfy both low-temperature fixing ability and high-temperature storage stability.